Because of the high electrical conductivity and high transmittance in the visible light region, transparent conductive films are used for solar cells, liquid crystal display elements, surface elements for organic electroluminescence, inorganic electroluminescence, etc., electrodes for touch panels, and the like, and also are used as heat ray reflection films for automobile windows or architecture, antistatic films, and various anti-fogging transparent heaters for freezer showcase and the like.
Here, examples of films known as the transparent conductive films include tin oxide (SnO2)-based thin films, zinc oxide (ZnO)-based thin films, indium oxide (In2O3)-based thin films, and the like.
As the tin oxide-based thin films, those containing antimony as a dopant (ATO) and those containing fluorine as a dopant (FTO) are commonly used. Meanwhile, as the zinc oxide-based thin films, those containing aluminum as a dopant (AZO) and those containing gallium as a dopant (GZO) are commonly used. The transparent conductive films most commonly used in the industrial field are based on indium oxide. Of these indium oxide-based transparent conductive films, indium oxide films containing tin as a dopant, i.e., In—Sn—O-based films, which are referred to as ITO (Indium tin oxide) films, are widely used especially because these films can be obtained easily as transparent conductive films having low resistance.
As a method for producing those transparent conductive films, a sputtering method such as direct-current sputtering or high-frequency sputtering is often used. The sputtering method is effective when a film is formed from a material having a low vapor pressure or when precise control of the film thickness is required, and is widely used in the industrial field, because the operation is very simple.
In the sputtering method, a target is used as a raw material of the thin film. The target is a solid containing a metal element which is to constitute the thin film to be formed. As the target, a sintered body of a metal, a metal oxide, a metal nitride, a metal carbide, or the like is used, or in some cases, a single crystal thereof is used. In the sputtering method, an apparatus having a vacuum chamber in which a substrate and a target can be placed is used, in general. After a substrate and a target are placed therein, the vacuum chamber is evacuated to high vacuum, and then the gas pressure inside the vacuum chamber is set to approximately 10 Pa or below by introducing a noble gas such as argon. Then, an argon plasma is generated by causing glow discharge between the substrate and the target where the substrate serves as an anode and the target serves as a cathode. The target serving as the cathode is bombard with argon cations in the plasma, and constituent particles of the target ejected by the bombardment are deposited onto the substrate to form a film.
Meanwhile, production of these transparent conductive films by the ion plating method is also studied. However, ITO films formed by the ion plating method have low resistance values. For example, when such an ITO film is used as a transparent electrode for a resistance-type touch panel, the thickness of the film has to be controlled to approximately 10 nm. Hence, it is very difficult to form such a film. In addition, as the panel size increases, it becomes difficult to control the variation in film thickness.
Here, indium oxide-based materials such as ITO are widely used for producing transparent conductive films as described above. However, indium metal is rare in the earth and is toxic, which raise concerns over adverse effects on the environment and the human body. For these reasons and the like, there is a demand for indium-free materials. As the indium-free materials, zinc oxide-based materials such as GZO and AZO and tin oxide-based materials such as FTO and ATO are known as mentioned above. Transparent conductive films are industrially produced from zinc oxide-based materials such as GZO and AZO by the sputtering method. However, these transparent conductive films are disadvantageous because of their poor chemical resistance (alkaline resistance and acid resistance) and the like. On the other hand, transparent conductive films of tin oxide-based materials such as FTO and ATO are excellent in chemical resistance, but it is difficult to produce a tin oxide-based sintered body target having a high density and a high durability. Hence, the transparent conductive films of tin oxide-based materials are disadvantageous in that these transparent conductive films are difficult to produce by the sputtering method.
In this respect, a Zn—Sn—O-based transparent conductive film is proposed as a material overcoming the above-described disadvantages. The Zn—Sn—O-based transparent conductive film is a material excellent in chemical resistance and hence overcomes the disadvantage of the zinc oxide-based transparent conductive film. As a Zn—Sn—O-based thin film, for example, a film is proposed which has a structure in which a transparent film made of a metal oxide of zinc and tin and a reflection film of chromium nitride are sequentially stacked on a glass substrate (see Patent Document 1). However, in Patent Document 1, the transparent film made of the metal oxide of zinc and tin is formed by a reactive sputtering method using a Zn—Sn-based alloy target, and hence the film characteristics of the formed transparent film are poorly reproducible. In addition, Patent Document 1 describes only the composition (Zn/Sn ratio) of the alloy target used, and does not describe the structure of the alloy target. In general, in a method for producing a metal oxide thin film by reactive sputtering using a metal target, the film composition and film characteristics vary remarkably, so that the yield tends to decrease. With a high direct current input power of an input power density of 2.0 W/cm2 or more, the variation in film characteristics is especially remarkable, and the productivity deteriorates.
Moreover, a method for forming a film by a high-frequency sputtering method using a Zn—Sn—O-based oxide sintered body target is proposed (see Patent Document 2). Patent Document 2 recites that when the crystal particle diameter of the Zn2SnO4 phase is in the range from 1 to 10 μm, the target is less likely to be broken during film formation. However, in mass production of sputtering targets, a sintered body having a crystal particle diameter as coarse as about 10 μm is not preferable, because the sintered body is highly likely to have cracks and fracture when handled in a process such as grinding. In addition, in the proposal of Patent Document 2, only a calcined powder is used as the raw material powder. Hard particles of the calcined powder do not collapse in a favorable manner during the pressing. Hence, the strength of the obtained compact is so low that the compact is highly likely to fracture during the transportation or the like of the compact. Therefore, the proposal of Patent Document 2 is unsuitable for mass production. In addition, in the proposal of Patent Document 2, a SnO2 crystal phase is present in the obtained oxide sintered body. As described in Patent Document 3 listed below, it is known that when a SnO2 crystal phase is present in an oxide sintered body, arcing occurs frequently during film formation under a condition of a high direct current power input (with an input power density of 1.764 W/cm2 or more). Moreover, occurrence of arcing with a high input power not only hinders stable formation of transparent conductive films having good characteristics, but also greatly deteriorates the productivity because there is no choice but to lower the input power value for suppressing the arcing. Hence, the proposal of Patent Document 2 is not preferable.
Under such a technical background, the applicant proposed a Zn—Sn—O-based oxide sintered body usable as a sputtering target or a tablet for vapor deposition such as ion plating for forming Zn—Sn—O-based thin films at high speed (see Patent Document 3).
Specifically, the Zn—Sn—O-based oxide sintered body is characterized by comprising a zinc oxide phase and a zinc stannate compound phase or comprising a zinc stannate compound phase, but containing neither a tin oxide crystal phase nor a tin oxide crystal phase in which zinc is dissolved to form a solid solution.
Note, however, that Patent Document 3 does not describe the resistance of the Zn—Sn—O-based oxide sintered body to thermal shock and the like. When the Zn—Sn—O-based oxide sintered body is used as a sputtering target or a tablet for vapor deposition without considering the resistance, cracks are formed in the Zn—Sn—O-based oxide sintered body during film formation by sputtering or ion plating, in some cases.
In addition, when cracks are formed in the Zn—Sn—O-based oxide sintered body serving as a sputtering target or a tablet for vapor deposition, not only the film characteristics of the produced transparent conductive films deteriorate and lose the stability, but also inevitable interruption of the film formation may result in great deterioration of the productivity.
In this respect, there is a demand for a Zn—Sn—O-based oxide sintered body which is resistant to breakage during the processing of the sintered body, which is also resistant to breakage and crack formation during the production of transparent conductive films (during film formation) when the sintered body is used as a sputtering target or a tablet for vapor deposition, and which enables high-speed and stable mass production of transparent conductive films without variation in film characteristics.